Is there data on how close to NDL undeserved hits occur?

[Kendall Raine]

Results from post dive sub-clavian self-doppler over roughly 400 man dives with five subjects (my regular dive buddies) doing OC mixed gas dives in the 200-350' range (TBT 10-25 minutes) showed that the following factors (not in order) associated with higher bubble grades:

1. Work
2. Cold
3. Emotional stress
4. Poor viz
5. Duration of BT
6. Prior rest-lousy sleep = higher bubble grades

Lots of auto correlations here. For example, deep dives in California and Michigan (viz:30', temp 40-60F) tended to have consistently higher grades than similar profiles in Florida (viz 80', temp 75F).

The deco profiles were done using a dual phase (modifed RGBM) curve-roughly equivalent to ZHL-16 C GF 10/90. Bubble grades ranged from I+ to III (K-M scale). No DCS occured on any of these dives despite some high bubble grades.

Not surprisingly, use of DPVs on deep dives lowered bubble grades for similar profiles.

As there have been no additional comments regarding this publication, I will share a few thoughts I had after reading it that I jotted down in the margins of the paper.

The study includes 2,629 divers and 39, 099 dives. They had 320 cases of DCS, for a rate of 0.82% or 82 cases/10,000 dives. This is significantly higher than the rate of somewhere around 1-3 cases/10,000 dives in recreational dives usually cited. There is no discussion of this rate. (see comments in posts 33, 35 and 36 and 38)

Nearly 74% (236/320) of the cases of DCS occurred in divers with a GF of 0.7-0.9. Unfortunately, there are no denominators for the GFs. I would not be surprised if these GFs were also the most common in all of the dives.

As per @dmaziuk it was not surprising that most cases of DCS implicated the medium compartments.

There is interesting information here, as often, to be taken with a grain of salt.

As for the Maroni paper, I found the following paragraph of particular interest:

"Another intriguing case is the effect of visibility on bubbles and DCS; our data in fact show that high visibility increases bubble formation (by an increase of depth, time, and GF facilitated by the good diving condition) but DCS prevalence is higher with low visibility. This also seems to indicate that even in the presence of lower bubble grades, the stress effect induced by low visibility, may increase deco-stress and bubble susceptibility."

The idea that emotional stress (poor viz) predisposes to DCS is not a new idea, but one about which not much has been studied (AFAIK-study references most welcome). The inverse correlation of bubble grades and DCS is also very interesting. Think about how this might work in the context of higher DCS incidence with first dives (as opposed to multiple dives in a series), even in non-provocative (recreational) dives and the implications for cold water (often also poor viz) dives (West Coast, Northeast, Midwest) particularly for students/infrequent divers, suggests that added conservatism (shorter TBT, TRT, and lower GF High) should be considered based on this factor.

It also adds an interesting dimension to the idea of acclimization and whether there is more to that than complement system depletion over a dive series.

 

[lowwall]

Results from post dive sub-clavian self-doppler over roughly 400 man dives with five subjects (my regular dive buddies) doing OC mixed gas dives in the 200-350' range (TBT 10-25 minutes) showed that the following factors (not in order) associated with higher bubble grades:

1. Work
2. Cold
3. Emotional stress
4. Poor viz
5. Duration of BT
6. Prior rest-lousy sleep = higher bubble grades

Lots of auto correlations here. For example, deep dives in California and Michigan (viz:30', temp 40-60F) tended to have consistently higher grades than similar profiles in Florida (viz 80', temp 75F).

The deco profiles were done using a dual phase (modifed RGBM) curve-roughly equivalent to ZHL-16 C GF 10/90. Bubble grades ranged from I+ to III (K-M scale). No DCS occured on any of these dives despite some high bubble grades.

Thanks for the data. Very interesting.

10/90 equivalent? I guess this was done a few years ago? Please repeat that using something that gives a shallower first stop, but similar total ascent times (40/80?)

 

[Kendall Raine]

Thanks for the data. Very interesting.

10/90 equivalent? I guess this was done a few years ago? Please repeat that using something that gives a shallower first stop, but similar total ascent times (40/80?)

You're right, it was a few years ago .

 

[scubadada]

Hi @Kendall Raine

Your comments on emotional stress are interesting. In a recent discussion of gas consumption, I mentioned psychological stress being a possible factor for me.

"Exertion and getting cold are the only two easily identifiable factors increasing my gas consumption. Psychological stress probably also plays a role but is harder to identify or quantitate."

There are many factors that may contribute to psychological/emotional stress other than poor visibility; current, surge, new location, difficult route, perception of danger, new or unskilled buddies... Some of these may contribute to exertion in addition to the psychological/emotional stress. The stress likely affects individuals very differently.

 

[L13]

Thanks for the data. Very interesting.

10/90 equivalent? I guess this was done a few years ago? Please repeat that using something that gives a shallower first stop, but similar total ascent times (40/80?)

Or 60/70 to be even more conservative

 

[boulderjohn]

Settings like 10/90 and 20/80 were used during the peak of the deep stop craze. The reference here is to a "modified" RGBM, and it has to be because even RGBM, very much a deep stop algorithm, does not stop that deep.

 

[Stoo]

There are other personal factors (eg obedity, dehydration, alcohol consumption, above mentioned PFO…) that contribute to this. Make it very difficilt to analyze low frequency events…

There are so many variations in a large sample of divers that I think it's pretty much impossible to draw any accurate conclusions.

The good news is that by adding more time in the shallows can pretty much eliminate most risks. Safety stops are lovely, but if you like what 3 minutes does for you, try 10 minutes, or more, putzing around in 20'.

 

[boulderjohn]

Safety stops are lovely, but if you like what 3 minutes does for you, try 10 minutes, or more, putzing around in 20'.

Unless you were already bent when you got to the safety stop depth, hanging around there long enough can pretty much guarantee you won't get bent.

 

[Stoo]

Unless you were already bent when you got to the safety stop depth, hanging around there long enough can pretty much guarantee you won't get bent.

That too... the pause that refreshes.

While we're at it, let's mention the need to really take that last 10' - 20' ascent from the safety/deco stop REALLY slowly. Nothing like taking a totally lovely profile and trashing it all by hammering the power inflator at 15' and rocketing up from there. Those last few feet matter more than most.

 

[Kendall Raine]

Settings like 10/90 and 20/80 were used during the peak of the deep stop craze. The reference here is to a "modified" RGBM, and it has to be because even RGBM, very much a deep stop algorithm, does not stop that deep.

Actually the bigger mods were extending the 100% stops from straight RGBM. We found adding a few minutes to the 20 plus 5 fpm ascent from the 20 up reduced VGE a lot. What we didn't try (wish we had) was using shallower first stops. In the end, over 400 man dives nobody got bent, niggles, or even fatigue so, at least for bounce dives (20-30 minute TBT), the curves seemed to work.

That said, I now use 55/70